Download ASC-176: Core Vaccination Program and Infectious Disease Control

ASC-176
Core Vaccination Program and
Infectious Disease Control for Horses
Fernanda Camargo-Stutzman and Kristen Harvey, Department of Animal Sciences
Introduction
Programs for the control of infectious
diseases are important components of
good management practices directed toward maximizing the health, productivity, and performance of horses. Infectious
diseases occur when horses are exposed
to a sufficient quantity of an infectious
agent that overcomes the horses' previously acquired resistance.
Some of the goals of disease control
programs include the reduction of factors that may decrease resistance or
increase susceptibility of disease in the
horse. Such factors include stress, overcrowding, parasitism, poor nutrition,
inadequate sanitation, contaminated
water source/supply, concurrent disease,
stringent exercise program, no turn-out
time, transportation, inclement weather,
weaning, disruption of established social groups, and movement of people,
vehicles, and/or equipment on and off
facilities during infectious disease outbreaks. Consistent utilization of good
management programs will lower the
incidence and/or severity of infectious
diseases in any particular herd.
A well-delineated vaccination program can also enhance resistance to
those diseases; however, vaccines can
protect horses from some, but not all,
infectious diseases. A horse or groups
of horses that are well fed and well managed are more likely to maintain good
health.
The occurrence of infectious diseases
tends to increase at sales or boarding
facilities, breeding farms, barns of performance and show horses, fairgrounds,
or racetracks. Such situations are ideal for
introducing and transmitting infectious
Show grounds and racetracks are examples of horse gatherings where infectious diseases can
quickly spread.
diseases, particularly infections of the
respiratory tract.
In farms where there is a resident herd
and an itinerant herd, the management
of the two distinct populations needs to
be taken into account. The introduction
of horses from various backgrounds and
origins and the commingling of horses
of different ages, career/use, and traveling status create special problems and
need to be specifically addressed in the
practice of disease control.
Management recommendations are
based on the practices of (1) segregation
of horses on the premises into small
groups, (2) the maintenance of each horse
group as an isolated unit, and (3) stress
reduction. Horses should be segregated
into groups that are similar in age, gestation status, use, and frequency of removal
of individual animals. Restrictions should
be placed on movement of horses into
and out of each established group, and
contact with transient horses in particular should be avoided. The greatest danger lies in the introduction of new horses
into established groups, especially those
with recent opportunities for exposure
to large, intermingled gathering of horses
from diverse sources (sales, shows, board-
ing stables, racetracks, competitions,
training centers, etc.).
Ideally, horses should be appropriately
vaccinated at least one month prior to
entering or leaving such a facility in order
to produce adequate antibodies before
the anticipated exposure. Ideally, before
a new horse is introduced into a closed
group, it should be isolated for 21 days.
In addition, a horse temporarily removed
from a group for prolonged transport or
contact with other horses (e.g., breeding, showing, training, veterinary care,
sales, etc.) should also undergo a 21-day
isolation period and evaluation for signs
of infection before being returned to its
resident group.
In cases of respiratory and digestive
system diseases, infection often spreads
rapidly, resulting in significant economic
impact. Immediate intervention with
infectious disease control measures is
required for successful containment
of the infection. Implementing the
practices of isolation, quarantine, and
disinfection is imperative for managing
an outbreak. Clinically ill horses should
be removed from their resident group
and kept in physical isolation away from
the remainder of the horse population. A
zone of empty space sufficient to serve as
a barrier should surround the isolation
area. Human and horse traffic through
the isolation area should be restricted.
Personnel caring for horses placed in
isolation should wear protective outer
clothing, disposable latex gloves, and
footwear that can be immersed in a
disinfectant footbath. All of the outer
protective clothes should be removed
upon leaving the isolation area. Ideally,
staff handling horses in isolation would
be assigned to that group only. Remember to thoroughly clean and disinfect
the area used as isolation facilities after
resolution of the disease.
Vaccination
Vaccination is also an important part
of infectious disease control.
The American Association of Equine
Practitioners (AAEP) and the American
Veterinary Medical Association (AVMA)
divide their vaccination program into
two categories: core and risk-based vaccination guidelines. The AVMA defines
core vaccinations as those “that protect
from diseases that are endemic to a region, those with potential public health
significance, required by law, virulent/
highly infectious, and/or those posing a
risk of severe disease.” It is recommended
that all horses receive the vaccinations included in this group. See tables below for
specific age or gestation status before vaccinating your horses. Diseases included
in the core list are tetanus, rabies, eastern
and western equine encephalomyelitis,
and West Nile virus. Diseases included
in the risk-based list vary regionally,
from population to population within an
area, or even between individual horses
within a population. Diseases included in
the risk-based list are anthrax, botulism,
equine herpesvirus (rhinopneumonitis),
equine viral arteritis, equine influenza,
Potomac horse fever, rotaviral diarrhea,
and strangles. It is important to consult
your veterinarian to develop a vaccination protocol for your horse population.
This publication will focus on the core
vaccination guidelines.
Core Vaccination Guidelines
Tetanus
Tetanus is caused by the toxins of the
anaerobic bacterium Clostridium tetani.
C. tetani is a ubiquitous soil inhabitant
and can be isolated from feces of horses
and other domestic animals. Among
animals, the horse is considered the most
sensitive to tetanus toxin. Tetanus is an
infectious disease but not a contagious
disease, and, as such, it cannot spread
from animal to animal or from animal
to humans.
The most common route of infection
is puncture wounds contaminated with
manure and soil that contain C. tetani
spores. However, causative wounds may
not be found in up to 30% of tetanus
cases.
C. tetani needs an anaerobic environment to multiply and produce toxins,
and that is why deep penetrating (or
puncture) wounds produce the most
favorable circumstances for the organism
to flourish and produce the toxins that
will lead to the disease. However, tetanus
can develop after minimal superficial
damage, provided an anaerobic state
develops in the affected area.
Tetanus toxins act in a way that all
muscles in the body contract simultaneously when stimulated, in contrast to the
normal state where contraction of one
muscle is accompanied by relaxation in
its counterpart. This shows up clinically
as muscle rigidity and muscle spasms.
The force of these contractions may cause
rhabdomyolysis (the breaking down of
muscles), intramuscular hemorrhage,
fracture of long bones, etc. The spasmodic (called tonic-clonic) convulsions
are very strenuous to the affected animal,
and the resulting hemodynamic instability is an important cause of death.
Clinical signs of tetanus include
rigidity, stiff and stilted gait, elevated
tail, persistent protrusion of the third
eyelid, flared nostrils, excessive reaction
to sound and light, profuse sweating,
and a “sawhorse” stance with legs in a
rigid extended position. As the disease
progresses, the horse develops “lockjaw”
2
(sardonic smile, or risus sardonicus)
where the jaw muscles are rigidly contracted. The horse will be unable to drink
or eat. If untreated, the involvement of
the respiratory muscles and the generalized musculoskeletal injuries will lead to
death in more than 80% of cases.
The diagnosis of tetanus is based on
clinical signs. Treatment involves the
provision of a safe and quiet environment, elimination of unbound toxin (by
administration of anti-toxin), sedation
and muscle relaxation, and general support (intravenous fluids, antibiotics, etc).
In some cases, euthanasia needs to be
considered.
Tetanus has long been preventable by
vaccination with inexpensive tetanus toxoid. These vaccines induce long-lasting
immunity. Primary vaccination involves
the administration by intramuscular
injection of two initial doses (1 month
apart) and a yearly booster. For unvaccinated horses that sustain an injury,
an optional approach is to administer
the tetanus antitoxin and the vaccine
at the same time (in different muscles).
The antitoxin will provide immediate
protection that will last for up to 4 weeks.
The second dose of vaccine should be
administered 4 weeks later to complete
the primary series. Thereafter, the annual
boosters are imperative for protection.
Rabies
Rabies virus causes acute encephalitis
in all warm-blooded hosts including
humans, and the outcome is almost
always fatal. Although all species of
mammals are susceptible to rabies
virus infection, only a few species are
important as reservoirs for the disease.
Transmission of rabies usually occurs
through the saliva from an infected
animal's bite. The wildlife species that
serve as the natural reservoir for infection
vary between different parts of North
America and include raccoons, skunks,
foxes, badgers, and bats. Bites on horses
are more frequently on the muzzle, face,
and lower legs. The virus then reaches
the spinal cord and brain through the
Good management is imperative in the control
of infectious diseases.
Horses in poor body condition have a higher
chance of acquiring an infectious disease.
nerves and causes rapidly progressive and
fatal encephalitis. In 2006, there were a
total of 53 cases of rabies in horses in the
United States, reported by the Centers for
Disease Control. A total of 6,943 cases
of rabies in animals were reported for
2006.
The clinical signs range from strange
behavior, lameness, neurological deficits,
self-mutilation, fear, agressiveness, and
depression, etc. Clinical signs are progressive from onset until death, which
generally happens by day 10.
There is no definitive antemortem
(before death) test for rabies in horses.
Clinical signs are suggestive but nondiagnostic. The diagnosis is made by
postmortem evaluation of the brain of
the suspected animal.
There is no effective treatment for
horses (and other animals, vaccinated
or unvaccinated) with clinical rabies.
Horses with clinical signs of rabies should
be isolated to prevent human exposure to
the virus.
One of the most important things
about rabies is that it is considered a
zoonotic disease and, as such, is transmissible from animals to humans. Animals with suspected rabies should only
be handled by trained individuals who
have had the appropriate rabies vaccination.
All horses, regardless of breed, gender,
or use, should be annually vaccinated
against rabies. The unvaccinated horse
should receive a primary set of three
doses and annual boosters. Vaccinated
horses and broodmares should receive
annual boosters.
Eastern and Western
Equine Encephalomyelitis
(Sleeping Sickness)
Sleeping sickness is a lay term used to
describe a neurological disease caused by
viruses of the Alphavirus genus. These
viruses, causing agents of the Eastern
and Western and Venezuelan equine encephalomyelitides (EEE, WEE, VEE), are
arboviruses, meaning that they are transmitted by bloodsucking insects, such as
mosquitoes. EEE and WEE receive their
names because of the geographical location where they were more frequently
associated with outbreaks in the United
States. VEE causes outbreaks of encephalitis in horses in Central America, South
America, Mexico, and occasionally in the
southern part of the United States.
Although widespread vaccination
has reduced the size and number of outbreaks of EEE and WEE in horses, the
impact of these diseases is still significant
because of the rapid and severe nature of
clinical signs and high mortality rate in
affected horses. EEE in the United States
has been detected in all states east of the
Mississippi River and in some western
states. In total, several hundred equine
cases and about 10 human cases are
reported each year.
The life cycle of these viruses involves
the transmission between birds, rodents.
and mosquitoes. Humans are also susceptible to these diseases when the virus
is transmitted to them by infected mosquitoes. Ticks and many species of mosquitoes are capable of virus transmission.
Of these encephalitides, WEE has the
lowest mortality in horses (approximately
3
50%). EEE is the most virulent for horses,
with mortality approaching 90%.
VEE is a reportable foreign animal
disease. As such, vaccination against VEE
is not recommended in most parts of the
United States, including Kentucky.
The clinical signs for these diseases are
those of central nervous system (CNS)
involvement, including ataxia, paralysis,
irregular gaits, grinding of teeth, incoordination, circling, staggering, head
pressing, partial or total blindness, head
hanging low with drooping ears and lips,
and ultimately stupor. The profound depression associated with these infections
give rise to the common name of “sleeping sickness.” The course of disease varies
between 2 and 14 days. Nearly all horses
with EEE will die, whereas horses with
WEE have a higher chance of survival.
The diagnosis is made by analyzing the
cerebrospinal fluid (CSF) of the affected
horse or isolating the virus from the brain
of the dead horse. Blood samples are not
useful for virus recovery because usually
no circulating virus is present when signs
of encephalitis appear. It is not possible
to diagnose EEE or WEE by the clinical
signs alone, and currently there are no
reliable antemortem specific diagnostic
tests available. West Nile Virus
The family Flaviviridae, to which the
West Nile virus (WNV) belongs, has
the distinction of containing some of
the most important human pathogens
in the world. Some of the viruses in this
family include the causing agents for yellow fever, dengue, Japanese encephalitis,
tick-borne encephalitis, louping ill, etc.
Table 1. Core vaccination chart for adult horses (adapted from the AAEP).
Broodmares
Vaccinated:
Adult Horses (>1 year of age)
Vaccinated:
Unvaccinated:1
Unvaccinated:1
Tetanus
Annual, 4-6 weeks • 2-dose series
prepartum
• 2nd dose 4-6 weeks after 1st
dose
• Revaccinate 4-6 weeks
prepartum
Tetanus
Annual
EEE/WEE
Annual, 4-6 weeks • 2-dose series
prepartum
• 2nd dose 4 weeks after 1st
dose
• Revaccinate 4-6 weeks
prepartum
EEE/WEE
Annual, spring,
prior to onset of
vector season
WNV
Annual, 4-6 weeks It is preferable to vaccinate
prepartum
unvaccinated mares when
open. In areas of high risk,
initiate primary series as
described for unvaccinated
adult horses.
WNV
Annual, spring,
prior to onset of
vector season
Comments
• 2-dose series
• 2nd dose 4-6 weeks after 1st
dose
• Annual revaccination
• 2-dose series
• 2nd dose 4-6 weeks after 1st
dose
• Revaccinate prior to the onset
of the next vector season
Inactivated vaccine:
• 2-dose series
• 2nd dose 4-6 weeks after 1st
dose.
• Revaccinate prior to the onset
of the next vector season
Recombinant canary pox
vaccine:
• 2-dose series
• 2nd dose 4-6 weeks after 1st
dose
• Revaccinate prior to the onset
of the next vector season
Tetanus
Booster at time of penetrating
injury or prior to surgery if last dose
was administered over 6 months
previously.
EEE/WEE
Consider 6-month revaccination
interval for:
• Horses residing in endemic areas
• Immunocompromised horses
WNV
When using the inactivated or the
recombinant product, consider
6-month revaccination interval for:
• Horses residing in endemic areas
• Juvenile (<5 yrs of age)
• Geriatric horses (>15 yrs of age)
• Immunocompromised horses
Flavivirus chimera vaccine:
• Single dose
• Revaccinate prior to the onset
of the next vector season
Rabies
Annual, 4-6 weeks Annual, 4-6 weeks prepartum
prepartum
OR
OR
prior to breeding
prior to breeding
Rabies
Annual
Single dose, annual
revaccination
Rabies
Due to the relatively long duration
of immunity, this vaccine may
be given post-foaling but prior
to breeding and thus reduce the
number of vaccines given to a mare
prepartum.
1 Unvaccinated or lacking a vaccination history.
West Nile virus is the leading cause
of arbovirus encephalitis in horses and
humans in the United States. Since 1999,
more than 25,000 cases of WNV encephalitis have been reported in U.S. horses.
The occurrence of more than 3,000 human cases in 2007 indicates widespread
viral activity in the environment.
This virus has been identified in all
of the continental United States, most
of Canada, and Mexico. Several Central
and South American countries have also
identified WNV within their borders.
The virus is transmitted from avian
reservoir hosts by mosquitoes (and infrequently by other bloodsucking insects) to
horses, humans, and a number of other
mammals. West Nile virus is transmitted
by many different mosquito species, and
this varies geographically. Horses and
humans are considered to be dead-end
hosts for WNV, which means that the
virus is not directly contagious from
horse to horse or horse to human. Indirect transmission via mosquitoes from
infected horses is unlikely as these horses
do not circulate a significant amount of
virus in their blood.
The mortality rate in affected horses
is 30 to 35%. Many horses will improve
within 3 to 7 days of displaying clinical
signs. If the horse demonstrates signifi-
4
cant improvement, full recovery can be
expected within 1 to 6 months. Some
horses will persistently present residual
weakness, ataxia, gait, and behavioral
abnormalities, etc.
The clinical signs of WNV infection
are that of CNS involvement, which
includes change of personality, hyperexcitability, depression, aggressiveness, etc.
Some horses can show signs of sudden
sleep-like activity resembling narcolepsy.
Muscle twitching is also observed and
can involve all four limbs and trunk,
which will affect normal activities such
as walking, eating, and interaction with
handlers. Some horses present a short,
Table 2. Core vaccination chart for foals (adapted from AAEP).
Foals and Weanlings (< 12 months of age)
Of mares vaccinated in
the prepartum period:1
Of unvaccinated mares:
Comments
Tetanus
3-dose series:
• 1st dose at 4-6 months of age
• 2nd dose 4-6 weeks after the 1st dose
• 3rd dose at 10-12 months of age
Tetanus
3-dose series:
• 1st dose at 1-4 months of age
• 2nd dose 4 weeks after the 1st dose
• 3rd dose 4 weeks after second dose
Tetanus
EEE/WEE
3-dose series:
• 1st dose at 4-6 months of age*
• 2nd dose 4-6 weeks after 1st dose
• 3rd dose at 10-12 months of age prior to the
onset of the next vector season
EEE/WEE
3-dose series:
• 1st dose at 3-4 months of age*
• 2nd dose 4 weeks after 1st dose
• 3rd dose at 10-12 months of age prior to the
onset of the next vector season
EEE/WEE
Note: Primary vaccination series scheduling
may be amended with vaccinations
administered earlier to younger foals that are
at increased disease risk due to the presence
of vectors.
* Foals in the Southeastern U.S.: The primary
vaccination series should be initiated with
an additional dose at 3 months of age due to
early seasonal vector presence.
* Foals in the Southeastern U.S.: The primary
vaccination series should be initiated at 3
months of age due to early seasonal vector
presence.
A foal born during the vector season may
warrant beginning vaccination at an earlier
age than a foal born prior to the vector season
WNV
Inactivated vaccine:*
3-dose series:
• 1st dose at 4-6 months of age
• 2nd dose 4-6 weeks after 1st dose
• 3rd dose at 10-12 months of age prior to the
onset of the next vector season
WNV
Inactivated vaccine:*
3-dose series:
• 1st dose at 3-4 months of age
• 2nd dose 4 weeks after 1st dose
• 3rd dose at 10-12 months of age prior to the
onset of the next vector season
WNV
Note: Primary vaccination series scheduling
may be amended with vaccinations
administered to younger foals that are at
increased risk of exposure due to the presence
of vectors.
* Foals in the Southeastern U.S.: Due to early
seasonal vector presence, the primary
vaccination series should be initiated earlier
with the addition of a dose at 3 months of age.
Recombinant canary pox vaccine:
3-dose series:
• 1st dose at 5-6 months of age
• 2nd dose 4 weeks after first dose
• 3rd dose at 10-12 months of age prior to the
onset of the next vector season
Flavivirus chimera vaccine:
2-dose series:
• 1st dose at 5-6 months of age
• 2nd dose at 10-12 months of age prior to the
onset of the next vector season
* Foals in the Southeastern U.S.: Due to early
seasonal vector presence, the primary
vaccination series should be initiated at 3
months of age.
Recombinant canary pox vaccine:
3-dose series:
• 1st dose at 5-6 months of age
• 2nd dose 4 weeks after 1st dose
• 3rd dose at 10-12 months of age prior to the
onset of the next vector season
Flavivirus chimera vaccine:
2-dose series:
• 1st dose at 5-6 months of age
• 2nd dose at 10-12 months of age prior to the
onset of the next vector season
Rabies
3-dose series:
• 1st dose at 6 months of age
• 2nd dose 4-6 weeks after 1st dose
• 3rd dose at 10-12 months of age
Rabies
3-dose series:
• 1st dose at 3-4 months of age
• 2nd dose 4 weeks after 1st dose
• 3rd dose at 10-12 months of age
A foal born during the vector season may
warrant initiation of the primary vaccination
series at an earlier age than a foal born prior to
the vector season.
There are no data for the use of the
recombinant or chimera product in foals
< 5 months of age. If either product is
administered to foals at < 5 months of age, the
recommended primary schedule should still
be completed.
Rabies
1 Against the disease indicated.
slow, stilted gait which sometimes is confused with lameness. They can also show
signs of ataxia and incoordination. They
may collapse and be unable to get up,
requiring aggressive supportive care.
It is impossible to diagnose WNV from
clinical signs alone, as many other diseases
that affect the CNS present the same clinical signs. There are some laboratory tests
that can be run in an attempt to diagnose
WNV, including complete blood work, serum biochemistry, CSF analysis, and other
tests. There are also postmortem methods
for confirming WNV infection.
There are three types of West Nile
virus vaccines, so you need to discuss
with your veterinarian about which one
is the most appropriate for your herd. 5
Other considerations on arboviruses
include the use of mosquito repellent and
elimination of mosquito breeding sites
(water in flowerpots, pet bowls, buckets,
barrels, discarded tires, and other items
that could collect water).
References
American Association of Equine Practitioners (www.aaep.org).
Centers for Disease Control and Prevention, Division of Vector-Borne
Infectious Diseases, Eastern Equine
Encephalitis (http://www.cdc.gov/
ncidod/dvbid/Arbor/eeefact.htm).
Centers for Disease Control and Prevention, Division of Vector-Borne Infectious Diseases, West Nile Virus (www.
cdc.gov/ncidod/dvbid/westnile/).
Centers for Disease Control and Prevention, Rabies (http://www.cdc.gov/
RABIES/).
Gibbs, E.P.J., Long, M.T. Equine Alphaviruses. In: Equine Infectious Diseases.
Editors: Sellon D.C. and Long, M.T.
Saunders-Elsevier, Saint Louis, Mo.,
pp. 191-197, 2007.
Long, M.T. Flavivirus Infections. In:
Equine Infectious Diseases. Editors:
Sellon, D.C. and Long, M.T. SaundersElsevier, Saint Louis, Mo., pp. 198-206,
2007.
MacKay, R.J. Tetanus. In: Equine Infectious
Diseases. Editors: Sellon, D.C. and
Long, M.T. Saunders-Elsevier, Saint
Louis, Mo., pp. 376-380, 2007.
United States Department of Agriculture,
Eastern Equine Encephalomyelitis
(http://www.aphis.usda.gov/lpa/pubs/
fsheet_faq_notice/fs_aheasterne.
html).
Wilkins, P.A., Del Piero, F. Rabies. In:
Equine Infectious Diseases. Editors:
Sellon, D.C. and Long, M.T. SaundersElsevier, Saint Louis, Mo., pp. 185-1191,
2007.
Wilson, W.D., Spensley, M.S. Preventive Medicine Programs. In: Current
Therapy in Equine Medicine 3. Editor:
Robison, N.E. W.B. Saunders, Philadelphia, Pa., pp. 35-50, 1992.
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